The present invention relates to a polishing machine for flattening surfaces of substrates, especially surfaces of semiconductor wafers on which device patterns are formed.
A polishing machine for flattening a surface layer of a semiconductor wafer on which devices are formed is disclosed in Japanese Unexamined Patent Publication (JP-A) No. 330261/1996. This machine has a turntable, known as a rotatable platen with a polishing cloth or texture adhered on the surface of the platen, and a wafer holder placed above the platen. A semiconductor wafer attracted to the bottom surface of the holder is placed in direct contact with the polishing cloth on the platen. A fluid (pressured air or water) applies pressure against the back side of the wafer. During a polishing process, both platen and holder are rotated to polish the surface layer of the semiconductor wafer. A diameter of the platen is larger than that of the wafer.
In the machine described above, the polishing operation is performed by pressing the semiconductor wafer against the polishing cloth on the platen and rotating the platen and the wafer. During this operation, the wafer is held to the bottom surface of the holder having a nearly same diameter of the wafer. Therefore, it is impossible for an operator to directly view the polishing surface of the semiconductor wafer. And most of polishing slurry supplied onto the polishing cloth on the platen is splashed by the centrifugal force created by the rotation of the platen. In consequence, about 30% amount of the slurry is loss. So it is desired to make effective use of the polishing slurry.
In the coming of the information-oriented era, a ceaseless high demand for high-level electronic devices and appliances is huge. Especially, it is expected that the demand for personal computers will be greatest among various devices and appliances. It is considered that the semiconductor industry will shift to the next-generation wafer fabrication process with this trend. In the future, 300-mm wafers or 400-mm wafers will be introduced. There is an urgent demand for the development of CMP (chemical/mechanical polishing) equipment for flattening the surface layers of devices formed on such larger wafers. Furthermore, it is necessary to develop polishing machines capable of polishing such larger-size bare silicon wafers.
It is considered that in CMP, the film thickness uniformity and surface flatness are the most important characteristics among quality characteristics. Namely, the flatness is the most important characteristic in bare silicon wafers.
Where larger wafers are polished by the conventional polishing method, if polish slurry is supplied by the conventional method (namely, the slurry is supplied on the platen through a pipe which is equipped outside the holder), it is difficult to make uniform the flow rate of polishing slurry across the total surface area of the wafer. Furthermore, even if wafer unduration or waviness is assumed to depend only to the viscoelastic characteristics of the polishing cloth, it is practically difficult to form the unduration of the wafer surface.
In addition, the flatness of the wafer can not be observed or detected by the existing polishing machine during polishing. One method heretofore proposed for detecting the flatness of the wafer during polishing is to measure variations in the load on an electric motor that drives the platen or polishing head (JP-A-138529/1993, JP-A-70751/1997, and JP-A-262743/1997). Another proposed method is to measure the reflection of the ray ejected on the film through the formed holes in the platen of the polishing machine (JP-A-309559/1993 and JP-A-160420/1998). However, none of them have been put into practical use.
Larger wafers are so expensive that is requires to diminish a loss. This requires to detect the flatness of each wafer during the polishing and the flatness must be modified to the desired degree of flatness of the wafer based on the obtained data. For this purpose, it is essential to control the flatness.
In the current polishing operation, the cost of polish slurry mainly dominates a large proportion of the variable cost. Indeed, the cost of slurry reaches 30% of the variable expenses. Furthermore, the efficiency of the slurry is only several percent and a decrease in the cost of slurry will reduce the amount of wastes. This will affect the environment greatly. Where slurry is supplied onto a large surface platen, limitations are imposed on the efficiency of utilization of the slurry. Therefore, there is an urgent demand for reducing the slurry cost.
The final problem arises from the fact that wafers are polished within a cleanroom, which is required to have a high degree of cleanliness. Of course, the cost per a unit area becomes expensive when the cleanroom is kept at high degree of cleanliness. This means that elements or members introduced into the cleanroom must inevitably be made compact.